1. Field of Invention
This invention concerns the accurate geolocation of emitters, as opposed to estimation of emitter range alone, using sensor-emitter angle-of-arrival (AOA) or azimuth rate measurements from single or multiple platforms. As used herein, geolocation refers to x,y coordinate position in a local level frame, where the local level frame is a plane tangent to the surface of the earth directly below the sensor position.
2. Related Art
Emitter azimuth or bearing measurements are typically made separated at points in space and time; hence this technique involves a discrete time state estimation problem. The sensor used to measure the angle rate may involve detection of RF (radio frequency) energy, such as a phase interferometer, an IR (infra red) tracker, or it may involve detecting acoustic energy, such as a sonar array. Spatially distributed sensors on multiple moving platforms may be used, or just a sensor on a single platform.
Measuring emitter azimuth is a statistical process that typically results in systematic and random errors corrupting the true azimuth value. Systematic errors are constant biases, or other errors exhibiting a long correlation time compared with the time over which the estimation is performed. Estimation techniques, such as those based on the Kalman filter, can very successfully convert azimuth measurements to range estimates when only random errors are present. However, systematic errors tend to degrade performance of the Kalman filter, as described for example in R. J. Brown and A. P. Sage, "Analysis of Modeling and Bias Errors in Discrete-Time State Estimation", IEEE Trans. Aerospace and Electronic Systems, vol AES 7, No. 2 March 1971. Also, M. Garvish and E. Fogel, "Effect of Bias on Bearings-Only Target Location", (IEEE Trans. on Aerospace and Electronics Systems, vol. 26, No. 1 January 1990) present an error bound, the Cramer Rao bound, describing the effect of biased bearing measurements on estimates of emitter location independent of the specific estimator realization used. The results are presented pictorially for a single platform in FIG. 1.
A bias error in bearing-only emitter location causes the estimate to lie on circle 16 through the actual emitter and sensor positions. As seen by that Figure, the magnitude of the bias effect is a function of emitter relative bearing from the observing platform 10. Even a small bias can significantly degrade range accuracy for small relative bearing from platform 10 to actual emitter location 12. Because range accuracy is degraded, emitter geolocation is rendered less accurate. At larger relative bearings, e.g., from platform 10 to actual emitter location 14, range accuracy is not degraded significantly by large bearing measurement biases. However, geolocation accuracy, as given by the Cramer Rao bound, will still suffer when the bearing systematic error is not negligible. Furthermore, aside from the degradation indicated by the Cramer Rao bound, the actual location algorithm used may fail catastrophically due to the unexpected bias.
A standard method of passively locating an emitter from a moving sensor is based on bearing or azimuth measurements made at different points along the sensor trajectory. This technique is referred to as the bearing method, or equivalently the bearing only method, and has been the topic of much research in the past. The effect of random errors on passive location system performance has been well understood for some time, e.g. as detailed in D. J. Torrieri, "Statistical Theory of Passive Location Systems", IEEE Transactions on Aerospace and Electronic Systems, T-AES vol.20, (March 1984), 183-198. However, actual implementations may have large systematic errors on the bearing measurements as well as random errors.
Systematic or bias errors typically arise when a trade-off is made in the system providing the bearing measurement used by the geolocation algorithm, such as a direction-finding (DF) system, involving system cost, weight and complexity. For instance, in RF sensor based systems, bias errors can be controlled by repeated real-time system calibration that compensates for thermal and frequency dependent errors. Calibration during installation can also compensate for direction-of-arrival or DOA dependent bias errors caused by sensor antennas and radomes. However, such compensation methods lead to complexities in system design, and can necessitate expensive testing during manufacture. Furthermore, calibration as a means to reduce bias error will not work for existing systems designed without incorporating that approach ab initio.
A further example of the cost, weight and complexity versus system bias trade-off that is made in the design and implementation of practical systems is the use of RF amplitude direction finding sensors as opposed to phase interferometer arrays. Wideband phase interferometers in practical RF systems have systematic errors associated with them on the order of tenths of a degree, but may require four or more antennas per DF measurement, as well as elaborate receiver designs to measure emitter signal wavefront phase between different antenna pairs. Amplitude DF systems may use as few as one antenna per DF measurement, and require less complex associated receivers, but can have systematic errors on the order of tens of degrees.
The largest component of the bias in the bearing measurement for such phase and amplitude DF systems is typically a function of the emitter angular location relative to the antenna boresites. Therefore, previous attempts at controlling the effect of bias have concentrated on maintaining the same relative boresite angle with respect to the antenna or antenna array.
Poirot and Arbid, "Position Location: Triangulation versus Circulation", IEEE T-AES, vol 14, no. 1, 1978, Mahapatra, "Emitter Location Independent of Errors in Direction Finders", IEEE T-AES, vol. 16, No. 6, 1980, and M. Mangel "Three Bearing Method for Passive Triangulation in Systems with Unknown Deterministic Biases", IEEE T-AES vol 17 (November 1981) 814-819, all describe techniques for handling such DOA dependent bias or systematic errors, as opposed to random errors. The approach in each case is essentially to fix the systematic error by maintaining the same relative bearing to the target. One disadvantage to these approaches is that the systematic error must be DOA dependent, and hence bias errors arising from internal system sources, such as errors in phase delay lines, will not be accounted for. Another drawback to these approaches is that a very precise observer track must be flown. Flying a precise track is operationally very restrictive for the sensor platform. Furthermore, these techniques do not allow the use of multiple observing platforms, each with its own systematic error.
There are techniques that do single platform ranging as opposed to geolocation using bearing rate of change, and so are insensitive to moderate azimuth bias. A. L. Haywood, "Passive Ranging by Phase-Rate Techniques" (Wright-Patterson AFB Tech. Report ASD-TR-70-46, December 1970), provides a theoretical analysis of this approach, and Kaplan, U.S. Pat. No. 4,734,702 provides one practical implementation. However, bearing-rate techniques estimate range, but cannot refine biased azimuth measurements to produce accurate emitter geolocation. Conventional geolocation systems utilizing bearing as an input, will not remove systematic errors in the bearing measurement either.
If the bias on the bearing or azimuth measurement is too large in such a conventional system, it is unable to produce an accurate geolocation estimate because the estimator diverges for reasons given in Brown and Sage, previously mentioned. FIGS. 2a-2d illustrate this behavior. The scenario used is an aircraft, such as an F-16, flying 10,000 ft at 480 kts, with the emitter at a frequency of 18 GHz, relative bearing of 45.degree., and distance initially of 123 nm. An interferometer with a baseline of 20" having negligible phase bias error is used to make the bearing measurements.
FIG. 2a shows the true and measured bearing with the measured bearing being input to the geolocation filter. The bearing measurement has only a very small systematic error. The estimator used is that described in T. L. Song and J. L. Speyer, "A Stochastic Analysis of a Modified Gain Extended Kalman Filter with Applications to Estimation with Bearings Only Measurements", IEEE Trans. on Automatic Control, vol. AC-30, October 1985. This estimator will be referred to as the MGEKF. Note that any other state-of-the-art bearings-only estimator would exhibit similar performance. FIG. 2b shows the accurate resulting MGEKF range estimate driven by the bearings shown in FIG. 2a.
FIG. 2c is the true and measured emitter bearings for the same scenario, but with a measurement phase bias error present. This phase bias error arises from receiver and delay line calibration errors. These introduce significant systematic error into the bearing measurement. FIG. 2d shows the degradation of the performance of the same MGEKF range estimate used in FIG. 2b, but now driven by the biased bearing measurements shown in FIG. 2c. The performance degration results from the presence of this systematic error.
Although the bearing-rate techniques do not suffer from this sensitivity to systematic error, which is largely eliminated in the process of differencing the measurement, they have another drawback, aside from producing range-only and inaccurate geolocation. This other drawback, which is significant, is that bearing-rate techniques cannot be used to perform emitter location utilizing sensors on multiple platforms since range is generated relative to the single sensor bearing rate of change.
Amplitude DF systems are widely used on many military aircraft to warn of illumination by radars on hostile missile-launch platforms. This application is referred to as radar warning or RW. RW systems generally use emitter signal amplitude/phase measurements and the known characteristics of frequency-independent antennas to generate DF.
There are several ways this is commonly done, e.g. two-channel amplitude and phase comparison, three channel amplitude and phase comparison, and single channel amplitude comparison. Two and three channel amplitude comparison have their DF measurement accuracies degraded principally by the following systematic errors: emitter signal polarization errors, antenna pattern and mechanical alignment inaccuracies, RF beam forming errors, and calibrated receiver imbalance errors. Thermal noise is the main source of random error. Single channel amplitude comparison systems do not have the RF beam-forming network errors, but experience larger polarization and antenna pattern errors, as well as larger thermal and receiver calibration errors.
Another DF measurement approach, phase comparison, or phase interferometry, also does not suffer beam-forming errors, and experiences smaller degradation from the other errors. Thus its DF measurement accuracy can typically be ten times better than single channel amplitude comparison, eight times better than two-channel amplitude and phase comparison, and four times better than three channel amplitude and phase comparison systems. Fully resolved phase comparison systems are typically needed for accurate geolocation, but such systems tend to be costly and complex. The complexity arises both from the small system phase measurement error needed, and accurate spatial resolution desired. Spatial resolution requires a long interferometer baseline, which in turn requires multiple antenna pairs to resolve the resulting phase or AOA ambiguities, as disclosed in U.S. Pat. No. 4,638,320 by Eggert et al. These antenna pairs then require receiver channels, which require calibration.
Phase comparison systems are not just complex and costly, but also suffer from operational difficulties. Amplitude comparison systems are well adapted to monopulse operation. But phase comparison systems, because of the multiple antenna pairs involved, typically require receiver channel switching, and so may not be fully resolved before the emitter signal disappears. The signal may disappear because of emitter scanning, terrain blockage, or sensor platform attitude changes. This is why amplitude comparison systems are typically used for threat or radar warning (RW), as opposed to phase comparison systems.
Unfortunately, DF measurements made with amplitude systems can be as inaccurate as 20.degree., and so are useless for doing precise emitter geolocation.